Smooth high tolerance porous tube and process for making

ABSTRACT

SEVERAL BENEFICIAL PROPERTIES ARE ACHIEVED BY CENTRIPETALLY MECHANICALLY WORKING AS BY ROTARY SWAGING OF POROUS METAL TUBING PARTICULARLY PREPARED FROM SINTERED POWDERED METALS. THESE INCLUDE EXCELLENT DIMENSIONAL TOLERANCE, EXCELLENT SURFACE FINISH, CONTROLLED DENSIFICATION DEPENDING ON THE AMOUNT OF REDUCTION, INCREASED STRENGTH AND YET THE OVERALL POROSITY OF THE TUBE IS RETAINED TO A VERY CONSIDERABLE EXTENT. THE INTERNAL SURFACE RETAINS ITS INITIAL ROUGH STRUCTURE.

United States Patent 3,626,744 SMOOTH HIGH TOLERANCE POROUS TUBE AND PROCESS FOR MAKING Frederick J. Sorgenfrei, Lake Elmo,Minn., assignor to Minnesota Mining and Manufacturing Company, St. Paul, Minn. No Drawing. Filed Aug. 28, 1969, Ser. No. 853,972

Int. Cl. B21k 21/00 US. Cl. 72367 8 Claims ABSTRACT OF THE DISCLOSURE Several beneficial properties are achieved by centripetally mechanically working as by rotary swaging of porous metal tubing particularly prepared from sintered powdered metals. These include excellent dimensional tolerance, excellent surface finish, controlled densification depending on the amount of reduction, increased strength and yet the overall porosity of the tube is retained to a very considerable extent. The internal surface retains its initial rough structure.

This invention relates to porous metallic tubes and particularly to porous metallic tubes having highly finished metallic outer surfaces, and sintered internal surfaces.

It is generally not possible to produce smooth accurately dimensioned uniform porosity tubing by normal powder metallurgy techniques. Short lengths of smooth tubing can be produced by die pressing and sintering. It is not possible, however, to produce long lengths of tube by die pressing and other powder metal techniques must be used. The sintered surface of materials made by these processes are usually very rough and it is difiicult to hold machine tolerances. There is also difiiculty in that they tend to smear and thereby lose surface porosity when machined as a result of the tangential application of forces. This problem becomes increasingly more difficult as the porosity (micronic rating) of the tube decreases. Several techniques have been proposed to facilitate the machining of porous materials without smearing the surface. All of these involve filling of the pores with a substance that can be removed after machining. For example, certain salts may be used to impregnate porous materials and, after machining or grinding is performed in the impregnated condition, the impregnant is removed. These are cumbersome high cost and multiple step processes and are very difficult to control, especially for long tubes. The present invention has as an object to finish and dimension the outer surface of porous sintered powdered metal tubing while maintaining uniform porosity to a substantial proportion of that of the unfinished tubing. The process of the invention provides a very inexpensive simple method of achieving these desirable results. The inner surface remains grainy or rough and the outer is dimensioned and finished to an extent determined by the extent to which the outer diameter is reduced.

The prior art shows that porous metallic tubes can be made and it has been suggested by Mott, US. Patent No. 3,313,621, that by insertion of a suitably shaped mandrel the tube can be hammered or swaged on the mandrel to shape the inside and outside. This process tends to reduce porosity quite sharply because both the outer surface and inner surface are subject to deformation. The mandrel in effect exerts a centrifugal force in opposition to the hammering.

It has now been found that centripetally mechanically working the outer surface simultaneously in two to three or more longitudinal zones simultaneously, as illustrated particularly by rotary swaging, is advantageously performed on porous sintered metallic tubing formed from metal powders until the diameter has been reduced to a desired extent. No mandrel is used. Unexpectedly the porosity is not greatly reduced when the outer or inner diameter is reduced by up to about 20-50%. This may be because there is no outward or centrifugal deformation of the inner surface. The increase in length usually associated with swaging operations is minimized by the process of the invention. Furthermore reduction in wall thickness is much lower than when a mandrel is employed.

The tubes used in this invention are preferably made from sintered powdered metals using conventional powder metallurgy techniques. Centripetal mechanical forming is then used on the outer surfaces at 2 to 4 positions or zones without applying opposite or centrifugal force directly to the inner surfaces. Thus swaging using rotary swaging procedures applies centripetal force and avoidance of a mandrel omits a centrifugal force. An excellent description of this metallurgical process (without respect to an internal mandrel) is found in Review of the Powder Metallurgy Process, July, 1966, published by the US. Army Production Equipment Agency, Manufacturing Technology Division, Rock Island Arsenal, Ill. Also see Mott, US. Patent Nos. 2,792,303 and 3,313,621. Rotary swaging is described in Metals Handbook, T. Lyman, editor, 8th ed. (1969), volume 4, pages 333 et seq.

The preferred powdered metals used in this invention are alloys such as austenitic chromium-nickel stainless steel. 'Ihese alloys generally containing 16.0 to 26.0 weight percent chromium, 6.0 to 22.0 weight percent nickel, 0.03 to 0.25 weight percent carbon, and occasionally some other elements are added to develop certain specific properties, such as 1.75 to 4.00 weight percent molybdenum or small amounts of titanium, tantalum, and mobium to minimize formation of chromium carbides, especially in welding. Standard types of these steels have been assigned numbers and specifications by the American Iron and Steel Institute. These are generally known in the art as stainless steels of the AISI series, types 301, 302, 304 and 305 generally referred to as 18-8 stainless steel, and the workhorse type 316 generally referred to as 18-8 M0. All of these AISI stainless steels of the 300 series are applicable in the practice of this invention. Of course, other ductile or malleable powdered metals can be used in fabricating the tubes used in this invention, such as nickel, iron, cobalt, copper, and the like, and alloys of such metals, including bronze, Monel, etc.

Filters are made from powdered metal which may vary widely in coarseness from as low as 20 to 30 or 35 microns up to about 1 mm. selected so that, upon sintering the resulting shaped article, the desired permeability, porosity or micronic rating is obtained. For purposes of making filter elements, it is preferred to use mesh sizes in the range of -20+325 (40-800 microns), such as 200+325 (40-72 microns), +200 (92 to microns), -50+100 (150 to 300 microns), --20+50 (300 to 800 microns) or blends thereof, suitably selected to produce the desired micronic rating or bubble point, and to that end small amounts, e.g., 1-20 weight percent, of -325 mesh 40 microns) or even -400 mesh 30 microns) powdered metals are blended with the coarse powder, i.e., with the -50+325 mesh (40-300 microns). The term mes referred to herein means mesh size according to US. Standard Sieve. Approximate closest sizes in microns are indicated parenthetically. The use of powdered metal with these mesh ranges will enable one to make tube structures which can be swaged in accordance with this invention with various micronic ratings, e.g., maximum beads passed in the range of 1 to 150 microns).

In fabricating each of the filter component layers, the powdered metal of desired mesh is blended with an )rganic heat-fugitive binder, such as those disclosed in US. Pat. Nos. 2,593,943 of Wainer; 2,709,651 of Gurnick at al.; and 2,902,363 of Joyner; the preferred binder is nethyl cellulose with which the lubricants used by Mott n U.S. Pat. No. 2,792,302 are unnecessary. Various sol- ICIIIS can be used in conjunction with these binders, such is Water, as well as various plasticizers, such as glycerin. The blending can be carried out in a conventional manner n various types of commercially available mixers, blend- :rs, tumblers, and the like, care being taken to insure :hat the blend is homogeneous and the components well lispersed. The resulting blend will be in the nature of a alastic mass or dough and will be similar in consistency to .hat of modeling clay. It is extruded by conventional nethods.

Sintering atmosphere, temperature, and duration of iintering depends upon the particular powdered metals ised and the selection of these conditions is within the ;kill of the art. In the case of the austenitic stainless steels nentioned above, a hydrogen or dissociated ammonia atmosphere with a dew point of 40 F. or lower and aintering temperatures in the range of 1200 to 1400 0, preferably 1250 to 1350 C., is suitable, and the duration )f sintering is usually from 10 minutes to 2 or 3 hours.

As is evident from the above, the porous tube is made :ntirely from powdered metals without requiring or employing wrought metal components or welding. Swaging s carried out on a rotary swagin-g machine of convenional type for example, the 2 die type illustrated in FIG. 1 page 334 or a 4 die type illustrated in FIG. 7 page 335 )f the above Metals Handbook Article. In general the rwaging operation is used as a finishing operation to provide close outside diameters as well as for the usual aurpose of decreasing sizes. The surprising feature is hat in this operation it is found that no internal mandrel s desirable and wall thickness is not greatly affected. \lso with moderate amounts of swaging or percentage de- :rease in diameter, porosity is decreased to much less :xtent than when a mandrel is used and there appears to )e no tendency for partial plugging of pores so that addiional etching steps are not needed.

The desired surface finish and porosity are produced y suitable combinations of mesh size of the starting naterial, green forming and sintering parameters and the lIl'lOLlIlt of reduction during swaging. The formation of the nitial tubes is not part of this invention and tubes having :alibrated porosities (bubble points) are obtained directly. \n example of formation is included solely for con- 'enience to readers hereof.

The final size and shape of the tube is determined by he size of the swaging die. Various shapes are illustrated n FIG. 8 of the Metals Handbook Article so that tapers, :ontours or points may be introduced if desired. Single )r multiple reductions can be made with or without an ntermediate annealing step if desired. All these will be vithin the skill of the art from the present disclosure.

The articles produced by the process of the invention, .e., high surface quality, close tolerance, porous tubes have many applications, for example, as frictionless air turns, as ilm de-curling bars, as web or film waters, as air clamps, :tc. The tubes can be used for air bearings, e.g., for iandling yarns or textiles or for applying lubricant to am after spinning. The lubricant can be forced through he porous tube and applied to the yarn. The smooth surace of the porous tube avoids damage to the yarn. Other pplications are in places where low friction tube or rod liding is involved, filters having fine micronic ratings microns absolute), fiow controllers, flow restrictors nd diffusers.

Although the practice of this invention is described vith respect to stainless steel, it is applicable to any porous nalleable material such as the copper based alloys, espezially brass and bronze. Other applicable metals are nickel .nd nickel alloys, especially superalloys as cupronickels nd Monels, cobalt and alloys thereof, other iron alloys including low alloy steels, precipitation hardening stainless steels and ferrous superalloys. Ductile reactive metals and alloys can also be used such as titanium, zirconium, niobium, tantalum and their alloys. Swaging of Group VI metals is difficult and must usually be done above room temperature. Aluminum is difficult to sinter, especially into tubular configurations but products made of it and its alloys can also be improved by this invention.

Porosity of tubing such as here described may be measured by ASTM Test E12861 or it may be estimated as to the largest pores by the Bubble Point test described in the report of Micro Metallic Corp., Development of Filters for 400 F. and 600 F. Aircraft Hydraulic Systems, WADC TR 56-249. Pressure drop across the porous surface measured in suitable units at various rates of fiow is subject to the difficulty that the capacity of a long porous tube may not be reached at feasible flow rates.

Tubes may be open at both ends if desired or closed at one end. The following shows how such a tube may be made to be swaged in the process of the invention. Preferably porosities will be in the range of from about 1 to microns with pressure drops less than 50 cm. of

mercury.

A clay-like mass is produced by first dry-blending 3.0 kg. of 316 L stainless steel powder of 100 to 200 mesh (92 to 150 microns) size and 150 grams of methyl cellulose and then blending with 600 cc. of 10% by volume glycerine in water for about 1 hour in suitable apparatus such as a Braeblender Sigma blade mixer. The clay-like mixture is extruded by conventional techniques using standard dies for the purpose. A suitable apparatus is a ton Loomis extrusion press. Pieces are extruded up to about 1.2 meters (4 feet) in length having outside diameters of about 0.51 inch (13 mm.) and internal diameters of about 0.31 inch (7.7 mm.). The extruded pieces are air-dried for 1215 hours and prefired at 2150 F. (1170 C.) for two hours in dissociated ammonia. A second quantity of the clay-like mass is extruded through a .327 inch (8.3 mm.) die as a rod. The rod is dried overnight and prefired at 2150 F. (1170 C.) for two hours. The rod or plug is isopressed at 35,000 psi. and is then inserted into one end of the tube. The assembled structure with suitable steel mandrel as a filler to prevent collapse is similarly isopressed, the mandrel is withdrawn and the tube is then sintered for two hours at 2460 F. (1350 C.) in a dissociated ammonia atmosphere. Variations in sizes of particles together with conditions of pressing and firing give tubes having various porosities.

Tubes of stainless steel having various porosities and lengths are reduced from about 0.410 inch (10.8 mm.) outer diameter to 0.376 inch (9.53 mm.) outer diameter using a rotary swaging machine to provide centripetal mechanical working and inch (-9.5 mm.) long dies with a partial taper. Similar results are attained using rotary swages with four dies affecting different longitudinal zones. The finished tubes are characterized by micronic rating and bubble point determined as described in the above-mentioned ASTM procedure and WADC report. Accurate measurements of internal and external diameters, are made and air is forced through the tubes at various rates and pressure drops are measured in centimeters of water or mercury depending on relative areas of flow involved. The data are summarized in Table 1. In tubes of the lengths and porosities of Examples III and VI closed at one end it is found that about 98% of the flow occurs in the proximal 12 inches and about 84% in the proximal 6 inches. The tubes of all examples with the possible exception of Example I may therefore be considered as being of approximately 12 inch effective length in the pressure drop tests. The relatively rough internal surfaces are characteristic of tubes prepared by this process. A deburring operation may be employed when ends have been cut.

TABLE 1 Pressure drop (cm. 1120') .Micronic rating from Inner Outer Air flow 2 Percent of bubble point (a) diameter diameter theoretical Example 111B=Before (A=After) finch) (inch) 40 81 121 162 density I 8.5 12.7 16.6 74 A 3 37.5 43 46 80 H 1 9. 6 12. 7 16. 7 03 14. 20.7 27.1 77 20.8 60 In 17. 9 41. 4 70 IV 6. 9 13. 5 22. 7 55 9.0 18.0 30.0 58 V L; 5.6 11.8 20.0 53 8.0 15.9 27.5 61.5 VI 1 8. 4 15. 0 23. 4 '1 19.5 32.5 47.7

1 In cm. of Hg where marked; otherwise in cm. of H20.

2 Air flow cubic feet per hour per tube; effective length about 12 inches. Tube increases 78.0 cm. (before) to 81.6 cm. after rotary swaglng.

b About 12 inches long initially.

e 34.5 inches long at end of swaging operation.

(1 40 inches long initially.

a 43 inches long initially.

EXAMPLE VII EXAMPLE VIII A filter tube about 150 cm. long with a bubble point of 12.5 cm. H O was swaged and the result was a tube with a bubble point of 15.8 cm. H O. The ultimate tensile strength went from an average of 13,200 p.s.i. before swaging to 20,500 psi. after swaging. The density changed from 53% to 61.5% of theoretical, yet porosity remained open and uniform.

EXAMPLE IX The diameter of a piece of open-ended porous tubing 52 inches long with a bubble point of 16 cm. Hg was measured at 2 inches intervals along the tube. The total deviation from nominal along the tube length was .0005 inch. A single point measured .0005 inch less than the rest of the tube. Porosity remained open and uniform. The diameter after swaging was 37475:.00025 inch.

EXAMPLE X This example was performed in part according to US. Pat. No. 3,313,621 with swaging on a mandrel. Porous stainless tubing in about 12 inch lengths of both coarse and fine micronic ratings were swaged with and without mandrels. Table 2 summarizes the results.

largest or maximum pores an increase means reduction in size of the largest pores but not necessarily proportional dimensional changes in all pores. The porosity is lower at higher bubble points and doubling thus corresponds approximately to halving overall porosity.

The finer porosity tube when swaged on a mandrel loses nearly all of its surface porosity (greater than cm. Hg pressure) and must be etched to be reopened. Those pores that still remain open are erratic as to their position. They are too few to permit of any reasonable gas flow.

EXAMPLE XI Tubes with coarse and fine micronic ratings as in Example X are reduced in diameter in a different embodiment of the process of the invention by rotating a section of the tube in a lathe so that 25 mm. (1 inch) diameter 0.7 cm. (0.25 inch) wide steel idler rollers are forced against it simultaneously and with about equal force thereby centripetally applying mechanical force in three longitudinal zones. The rolls are mounted on the tool post which is mechanically traversed so that the rollsare moved slowly axially along the rotating tube. Reduction in porosity is effected with no visible smearing of the surface due to tangential forces.

What is claimed is:

1. A process for finishing and dimensioning the outer surface of porous tubing of sintered powdered metal while retaining a substantial proportion of the porosity thereof which consists essentially of centripetally mechanically working the said outer surface in two to four longitudinal zones simultaneously and reducing the diameter of the said tubing to the desired dimensions whereby at least part of the outer surface between pores is brought to a high TABLE 2 Outer Inner Percent Percent Bubble Pressure diameter diameter wall theor. point drop 1 tinch) (inch) reduction density (cm. H2O) (cm. H2O) Coarse Material:

As siutered 135 204 53 16 4 Swaged without a manderl. B77 238 1.4 64 21. 5 8. 7 Swaged on a mandrel B82 .284 31. 0 70 38 42. 7

Fine material: (Cm. Hg)

As sintered .454 278 70 5 54 Swaged without a mandrel. .376 .182 +11 80 10 23 Swaged on a mandrel .385 250 24: 93 50 (11) 1 Pressure drop measured at an air flow of cubic feet per hour. Substantially impermeable to gas flow.

finish and the internal surface is reduced without blockage of pores or alteration of the internal surface finish.

2. A process according to claim 1 wherein mechanical working is effected by rotary swaging of at least a portion of an empty tube.

3. A process according to claim 2 wherein the tube is composed of stainless steel.

4. A process according to claim 2 wherein the stainless teel powder sintered in making the tube is from about 20 0 about 325 mesh.

5. A process according to claim 2 wherein the pore size )f the tube as measured -by the bubble point is reduced by lot more than about half.

6. A process according to claim 2 wherein the tube is :losed at one end.

7. A process according to claim 6 wherein the tube is :omprised of stainless steel.

working is efiected by rolling at least a portion of a tube with three rollers applied simultaneous with essentially equal force.

References Cited UNITED STATES PATENTS 2,628,516 2/1953 Brace 29-420.5

LOWELL A. LARSON, Primary Examiner US. Cl. X.R.

8. A process according to claim 1 wherein mechanical 10 29 42().5 

